Astragalus bisulcatus
Astragalus bisulcatus, commonly called two-grooved milkvetch or silver-leafed milkvetch, is a leafy perennial with pea-like flowers. It is native to central and western North America, and typically grows on selenium-rich soils. It accumulates selenium within its tissues, and when livestock consume it, the selenium can be toxic.
Description
Astragalus bisulcatus is a herbaceous perennial with a thick woody root-stock, growing 30 to 100 cm tall. It has numerous simple stems and long pinnate leaves. The flowers are produced in narrow elongated racemes. The flowers are white or purple in color and bloom in late spring and early summer. The seed pods characteristically are two grooved on the upper surface. The fruits ripen and release their seeds in July and August. The seeds are dark brown or black, reniform in shape, 4 mm long and 2 mm wide.Selenium hyperaccumulation
This plant very effectively takes up selenium from soil into all organs at a level of 704–4,661 mg/kg dry weight. Its symbionts include a nitrogen-fixing bacterium and a unique endophytic fungus that produced elemental selenium. Selenium-resistant moths eat its leaves. Selenium-resistant wasps parasitize the moths.The genetic basis for its ability to put large amounts of selenium into itself is still unclear as of 2011.
The genetic basis for its ability to tolerate large amounts of selenium in its body is known. The plant has a cysteine—tRNA ligase that is more resistant to mistakenly using selenocystine. This resistance is attributed to a single amino acid change in its catalytic pocket. E. coli and S. cerevisiae that have their cysteine—tRNA ligases edited to have an analogous change also become more resistant to selenium toxicity.